Turning A White Mud Cake Mix Into A Marble Mud Cake ??? Please Help!!

I am making a chocolate marble cake for a 40th birthday today but I could only find a "White Chocolate Mud" cake mix. The supplier told me to just add cocoa powder to part of the batter. This sounds all sounds very simple, but I still have some concerns.

1. Will this change the texture and affect the baking as the chocolate batter would be drier?

2. Will the chocolate part have enough chocolatey flavour?

3. How much cocoa do I use?

4. Could I use chocolate frosting in the batter instead of cocoa powder.... would this work??